This application claims the benefit of Korean Patent Application No. 2002-10466 filed on Feb. 27, 2002, in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) display device, and more particularly, to an organic electroluminescent display device having an improved light extraction efficiency of light generated by an organic layer.
2. Description of the Related Art
Generally, organic electroluminescent display devices are spontaneous emission display devices that emit light by electrically exciting fluorescent organic compounds. The organic electroluminescent display devices are expected to be applied to next generation display devices that can overcome shortcomings of liquid crystal display devices, with characteristics including a low driving voltage, enabling the production of thin and small display panels having a wide viewing angle, and a high response speed.
An organic electroluminescent display device using a layered structure has been developed by the Eastman Kodak Company, and has been commercialized as a green-emitting display device having an improved lifetime by the Pioneer Electronic Corporation. Recently, many kinds of new materials having various molecular structures that are advantageous in organic materials, and organic electroluminescent (EL) display devices for use as color display devices having excellent characteristics including a lower DC drive voltage, small and thin structure, and spontaneous emission, have been the subjects of active research.
An organic electroluminescent (EL) display device includes organic layers having a predetermined pattern on a glass or a transparent insulating substrate, and electrode layers formed on and under the organic layers. The organic layers are made of organic compounds. Usable materials of the organic compounds include copper phthalocyanine (CuPc), N,N-di(naphthalene-1-yl)-N,Nxe2x80x2-diphenyl-benzidine (NPD), and tris(8-hydroxyquinoline) aluminium (Alq3).
In the organic EL device having the above-described configuration, if a positive voltage and a negative voltage are applied to the electrodes, holes injected from the electrode to which the positive voltage is applied move to an emitter layer via a hole transport layer, and electrons from a corresponding one of the electrodes are injected into the emitter layer via an electron transport layer. The electrons and holes are recombined at the emitter layer to generate excitons. The generated excitons de-excite from an excited state to a base state so as to have fluorescent molecules of the emitter layer emit light, thereby forming an image.
The light efficiency of the organic EL device driven as described above is divided into an internal efficiency, and an external efficiency or an extraction (taking-out) efficiency. The internal efficiency depends on the photoelectric conversion efficiency of an organic luminescent material. The external efficiency depends on the refractive index of each organic layer constituting the organic EL device. In other words, in the organic EL device, light having an outgoing angle larger than a critical angle cannot be taken out because the light is reflected at an interface between the substrate and the electrode layer, or between the organic layer and the electrode layer.
FIG. 1 shows a conventional organic EL device, where light emitted from an organic layer 10 is transmitted to a transparent glass substrate 11 from an interface between the glass substrate 11 and an indium tin oxide (ITO) electrode 12. The light transmitting efficiency is represented by the following formula:
xc2xd(Nout/Nin)2 
where N is a refractive index.
Based on the above formula, the light extraction efficiency for each color of the conventional organic EL device is shown in the Table 1 below.
As shown in the Table 1, as much light as 60% or more becomes extinct in the organic EL device due to a difference in refractive index between the ITO electrode 12 and the glass substrate 11.
An example of an organic EL device which prevents a reduction in light extraction efficiency is disclosed in Japanese Patent Publication No. 63-314795. The organic EL device includes a substrate having an ability to collect light, similarly to a projecting lens. However, the formation of such a projecting lens having the ability to collect light on a substrate is hard to accomplish in a device having a small pixel area, due to luminescence by an organic layer.
On the other hand, Japanese Patent Publication No. 62-172691 discloses an organic EL device comprising a first dielectric layer interposed between a transparent electrode layer and a light-emitting layer, and a second dielectric layer having a refractive index between the refractive index of a substrate glass and a refractive index of the light-emitting layer formed between the transparent electrode and the light-emitting layer.
Furthermore, Japanese Patent Publication No.1-220934 discloses an organic EL device having a lower electrode, an insulation layer, a light-emitting layer and an upper electrode formed on a substrate, and having a mirror which reflects light onto one side surface of the light-emitting layer.
However, since the film thickness of the light-emitting layer of the organic EL device is very small, the formation of a reflection mirror having a tapered surface on the side surface of the organic EL device is difficult using the currently available technique, and a high cost is required to perform the above formation.
The Japanese Patent Publication No. 11-283751 discloses an organic EL device characterized by making a diffraction grating or a zone plate into a component in an organic electroluminescence element which has one or more organic layer between an anode plate and a cathode.
In such an organic EL device, since irregularities must be formed on a surface of a substrate or a fine electrode pattern layer, or a separate diffraction grating must be provided, the fabricating process thereof becomes complex, making it difficult to attain an efficient productivity. Also, formation of an organic layer on the irregularities makes roughness severer due to the irregularities, thereby resulting in deterioration of durability and reliability of the organic EL device.
Accordingly, it is an object of the present invention to provide an organic EL device which can enhance the luminance of an image by reducing an internal light loss, so as to increase a light-emitting efficiency, and a method of manufacturing the same.
It is another object of the present invention to provide an organic EL device which can reduce a light loss using a light scattering effect at an interface between a layer having a low refractive index and a layer having a high refractive index, and a method of manufacturing the same.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above and other objects of the present invention, there is provided an organic EL device comprising a substrate layer, a first electrode layer formed on the substrate layer, an organic layer formed on the first electrode layer, and a second electrode layer formed on the organic layer, and a light loss preventing layer having areas with different refractive indices, formed between ones of the substrate, first electrode, organic, and second electrode layers having a large difference in refractive indices.
The light loss preventing layer may be formed between the substrate layer and the first electrode layer. Adjacent one of the areas having one refractive index may have a pitch in the range of 50 to 3,000 nm, and a thickness of the light loss preventing layer may be in the range of 0.01 to 50 xcexcm.
In a front-surface emission type organic EL device, the first electrode layer may be made of metal, and the second electrode layer may be made of indium tin oxide (ITO), wherein the light loss preventing layer is formed thereon.
The light loss preventing layer includes inorganic materials having different refractive indices and a difference in refractive indices may be in the range of 0.3 to 3.0. The inorganic materials may be two or more materials selected from the group consisting of SiOx (x greater than 1), SiNx (x greater than 1), Si3N4, TiO2, MgO, ZnO, Al2O3, SnO2, In2O3, MgF2, and CaF2.
To achieve the above and other objects of the present invention, there is also provided an organic EL device comprising a transparent substrate, a first electrode layer formed on the transparent substrate in a first predetermined pattern in a first direction and made of a transparent conductive material, an organic layer formed on the first electrode layer in a second predetermined pattern, a second electrode layer formed on the organic layer in a second direction at a predetermined angle with respect to the first direction, a light loss preventing layer provided between the substrate and the first electrode layer, wherein the light loss preventing layer has areas having different refractive indices, and is formed in a preset pattern, and an encapsulation layer which encapsulates the first electrode layer, the organic layer and the second electrode layer.
To achieve the above and other objects of the present invention, there is further provided an organic EL device comprising a transparent substrate, a pixel area having a transparent first electrode layer formed on the transparent substrate in a first predetermined pattern, an organic layer formed on the first electrode layer in a second predetermined pattern, an insulating layer formed over the transparent substrate which exposes a portion of the organic layer, and a second electrode layer formed on the organic layer and the insulating layer in a third predetermined pattern, a drive area having thin film transistors (TFTs) formed on the transparent substrate, wherein the drive area selectively applies a predetermined voltage to the first electrode layer, and a light loss preventing layer having areas having different refractive indices formed between the transparent substrate and the first electrode layer in a preset pattern.
To achieve the above and other objects of the present invention, there is still further provided an organic EL device having a thin film transistor (TFT), comprising a transparent substrate, a buffer layer formed on the transparent substrate, a thin film transistor (TFT) layer formed on the buffer layer, an insulating layer which covers the TFT layer, a first electrode layer formed on the insulating layer in a first predetermined pattern and to which voltages are selectively applied by the TFT, an insulating planarization layer having an opening which exposes a portion of the first electrode layer, an organic layer formed on the first electrode layer, a second electrode layer formed on the organic layer and the planarization layer in a second predetermined pattern, and a light loss preventing layer having areas having different refractive indices formed between the first electrode layer and the insulating layer, or on the second electrode layer in a preset pattern.
To achieve the above and other objects of the present invention, there is provided a method of manufacturing an organic EL device, the method comprising obtaining layers including a substrate layer, a first electrode layer formed on the substrate layer, an organic layer formed on the first electrode layer, and a second electrode layer formed on the organic layer, and providing a light loss preventing layer having areas with different refractive indices between ones of the substrate, first electrode, organic, and second electrode layers having a large difference in refractive indices, wherein providing of the light loss preventing layer comprises forming a first layer having a first component, masking the first layer using a mask having a predetermined pattern, injecting ions having a second component into the masked first layer, and heat treating the ion-injected layer under an oxidative atmosphere to provide the light loss preventing layer having an ion injected area and a non-ion injected area having different corresponding refractive indices.
To achieve the above and other objects of the present invention, there is provided another method of manufacturing an organic EL device, the method comprising obtaining layers including a substrate layer, a first electrode layer formed on the substrate layer, an organic layer formed on the first electrode layer, and a second electrode layer formed on the organic layer, and providing a light loss preventing layer having areas having different refractive indices, between ones of the substrate, first electrode, organic, and second electrode layers having a large difference in refractive indices, wherein the providing of the light loss preventing layer comprises forming a first layer by coating a composition including a photosensitive material and a binder resin, exposing the coated first layer in a predetermined pattern, and sequentially developing, bleaching and washing the exposed first layer.